Method for improving the imaging properties of a projection objective, and such a projection objective

ABSTRACT

The invention relates to a method for improving the imaging properties of a micro lithography projection objective, wherein the projection objective has a plurality of lenses between an object plane and an image plane, a first lens of the plurality of lenses being assigned a first manipulator (ml, Mn) for actively deforming the lens, the first lens being deformed for at least partially correcting an aberration, at least one second lens of the plurality of lenses furthermore being assigned at least one second manipulator, and the second lens being deformed in addition to the first lens. Furthermore, a method is described for selecting at least one lens of a plurality of lenses of a projection objective as actively deformable element, and a projection objective.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 11/915,191, filed on May 12, 2008, which is a National Phaseapplication under 35 U.S.C. §371 filed from International PatentApplication Serial No. PCT/EP2006/004929, filed on May 24, 2006, whichclaims priority to U.S. Provisional Application Ser. No. 60/685,716,filed May 27, 2005.

The invention relates to a method for improving the imaging propertiesof a microlithography projection objective.

The invention further relates to a method for selecting at least onelens of a plurality of lenses of a microlithography projection objectiveas actively deformable element for at least partially correcting animage defect.

The invention further relates to a projection objective.

Projection objectives are used in lithographic methods in order toproduce, for example, semiconductor components, image pick-up elements,displays and the like. In general, projection objectives are used forthe lithographic production of finely structured components.

A projection objective is constructed from a plurality of opticalelements that can all be lenses, or the projection objective can consistof a combination of lenses and mirrors.

The projection objective is used to image a structure or a pattern of amask (reticle), that is arranged in the object plane of the projectionobjective, onto a photosensitive substrate that is arranged in the imageplane of the projection objective. The structures or patterns to beimaged are becoming ever smaller in order to raise the integrationdensity of the components to be produced, and so increasingly higherdemands are being made of the resolution and of the imaging quality ofpresent-day projection objectives.

The imaging quality of a projection objective can be worsened byaberrations.

Such aberrations can be of various types. Thus, before such a projectionobjective is commissioned, aberrations can be imminent because ofunsatisfactory material specifications or inaccuracies in production orassembly. Such imminent aberrations can, however, be very largelyremoved during the production of the individual optical elements of theobjective and during the process of assembly, individual lenses of theobjective being provided with aspherized surfaces to this end, inparticular.

Aberrations can, however, also arise after commissioning and/or duringoperation of the projection objective or in the course of the ageing ofthe projection objective. Such aberrations can be caused byradiation-dependent variations in the optical material of the opticalelements of the projection objective. The radiation-dependent variationscan be permanent as, for example, in the case of compacting of thematerial of the optical elements, or they can be only temporary.Temporary variations in the optical material of the optical elements ofthe projection objective are overwhelmingly based on the fact that theindividual optical elements heat up and are therefore deformed duringthe exposure operation.

It is characteristic of radiation-dependent material variations whichinstitute aberrations that the two-fold symmetry of the rectangularfield of the illumination slit and of the image field is transferred tothe aberrations. This breaking of the rotational symmetry of theprojection objective leads to typical aberrations that are generallydifficult to correct.

Typical aberrations that are caused by heating of the material of theoptical elements which leads to changes in refractive index and also tosurface changes, or which are caused by changes in density (compacting)which can lead via changes in refractive index to wavefront errors are,for example, a constant-field astigmatism, a constant-field occurrenceof trefoil aberrations or a constant-field occurrence of quadrafoilaberrations. In addition to constant-field aberrations, however, thereis also the occurrence of aberrations that indicate a field dependenceor a field profile, for example a one-fold field profile of thedistortion (anamorphism) and an astigmatic field profile of the imagesurface.

It is known that a constant-field astigmatism can be corrected via theastigmatic deformation of a lens.

An actively deformable lens element that can be used to this end isdescribed, for example, in document WO 99/67683. Here, an activelydeformable lens is arranged in a mount, the lens being assigned amanipulator that has one or more actuators that act on the lens in afashion approximately perpendicular to the optical axis. The actuatorseffect forces and/or torques, which are not rotationally symmetrical anddeviate from the radial, on the optical element in order to producedeformations in the form of instances of bending. Depending on how manyactuators the manipulator has, it is possible to produce one-fold,two-fold, three-fold, or generally n-fold deformations or instances ofbending, in order correspondingly to correct one-fold, two-fold,three-fold or generally n-fold aberrations by deforming the activelydeformable lens at least partially.

By contrast with the abovementioned aspherizations of optical elementsthat can exhibit any desired complicated geometries, in order to correctcorrespondingly complicated wavefront error profiles, the correctionpotential of an actively deformable lens tends to be lower and isessentially limited to simple wavefront aberration profiles.

The conventional concepts of the use of actively deformable lenses arelimited to the use of a single actively deformable element which isintended to be used to correct a specific induced image defect. It iscertainly possible to use this mode of procedure to very largely correctthe image defect (aberration) to be corrected by appropriate deformationof the actively deformable lens, but this correction induces orintensifies other image defects, the result of this being that theoverall imaging quality of the projection objective is occasionally notimproved, or not substantially.

This means that there is still a need for a method for improving theimaging properties of a projection objective that can be used to combateffectively one or more image defects, in particular those that arecaused because of material ageing and/or temporary material heating.

It is an object of the present invention to provide such a method.

It is an object, furthermore, of the present invention to provide aprojection objective that is improved with regard to imaging properties.

It is, furthermore, an object of the present invention to specify for amicrolithography projection objective a method with the aid of which atleast one suitable lens can be selected from the plurality of lenses ofthe projection objective as actively deformable element for at leastpartially correcting an image defect.

In accordance with a first aspect of the invention, a method forimproving the imaging properties of a microlithography projectionobjective is provided, the projection objective comprising a pluralityof lenses between an object plane and an image plane, a first lens ofthe plurality of lenses being assigned a first manipulator for activelydeforming the lens, the first lens being deformed for at least partiallycorrecting an image defect, at least one second lens of the plurality oflenses furthermore being assigned at least one second manipulator, andthe second lens being deformed in addition to the first lens.

The method according to the invention is based on providing at least twoactively deformable lenses in a projection objective, and this opens up,inter alia, the possibility of correcting primary and higher orders ofimage defects independently of one another. In contrast, thispossibility does not exist when use is made only of one activelydeformable lens, this being based on the fact that primary and higherorders of image defects can be linearly dependent on one another. Forexample, the primary order of the two-fold image defect in Z5 islinearly dependent on the associated next higher order of image defectin Z12. The three-fold image defect in Z11 is, for example, linearlydependent on the next higher order in Z20, and the four-fold imagedefect in Z17 is linearly dependent on the next higher order in Z28. Inthis case, Z5, Z12, Z11, Z20, Z17, Z28 are Zernike coefficients whichare used as known to classify image defects in a series expansion of thewavefront.

The method according to the invention can be applied to projectionobjectives in an immersion configuration or in a dry configuration.Furthermore, the method according to the invention can be applied toprojection objectives that are not constructed exclusively fromrefractive elements, that is to say lenses, but can also be applied tothose that are constructed from a combination of refractive elements andreflective elements, for example mirrors.

A further advantage of the use of at least two actively deformablelenses for at least partially correcting image defects of a projectionobjective consists in that, given a suitable selection of the positionand/or the geometry of the at least two actively deformable lenses, itis possible to use simple manipulators to produce even more complicatedwavefront profiles, something which cannot be done, or can be done onlywith a substantially increased outlay, with the aid of only one activelydeformable lens, because of the limitation, explained above, of thedeformability of a lens.

Thus, the first lens and the at least second lens can preferably beadjacent or can be arranged at mutually optically conjugate sitesbetween the object plane and the image plane, both lenses then beingdeformed such that the deformations exhibit a different deformationprofile.

This constitutes an advantageous possibility of at least partiallycorrecting wavefront aberrations with a complicated field profile.

A similar effect can advantageously be achieved when the first lens andthe at least second lens are not adjacent or are arranged at notmutually optically conjugate sites between the object plane and theimage plane, and both are deformed such that the deformations exhibit adifferent deformation profile.

Whereas lenses that are arranged at adjacent or at conjugate positionsin the projection objective have substantially identical effects on thefield profile in the image, nonadjacent lenses or lenses in theprojection objective that are arranged at not mutually opticallyconjugate sites have different effects on the image field. In order inboth cases to compensate specific abberative field profiles orconstant-field aberrations, the at least two lenses are preferablydifferently deformed in the former case, and substantially identicallydeformed in the second case.

The abovenamed deformations of different profile can preferably beachieved by virtue of the fact that the first lens and the at leastsecond lens are of different shape.

If the at least two actively deformable lenses have a different shape orgeometry, even introducing identical forces exerted on the two lensesleads to different deformation profiles and thus to different correctiveeffects.

It is likewise possible to attain deformations of different profiles byvirtue of the fact that the first lens and the at least second lens aredeformed by introducing different forces.

In particular, the first and the at least second lens can be deformed byintroducing oppositely directed forces, in order to attain a specificcorrective effect.

When the first lens and the at least second lens are arranged at notmutually optically conjugate sites between the object plane and theimage plane, both can be at least substantially identically deformed,and yet the identical deformation can produce a different correctiveeffect on image defects in the image plane, since the corrective effectof two actively deformable lenses is a function of their position in theprojection objective.

Conversely, it is possible when the first lens and the at least secondlens are arranged at mutually optically conjugate sites between theobject plane and the image plane for both to be substantiallydifferently deformed in order in this way to attain a differentcorrective effect.

Furthermore, it can advantageously be provided to deform the first lensand the at least second lens at a specific ratio to one another.

If, for example, the first lens and the at least second lens influencethe wavefront aberrations in Z5 and Z12 in a constant fashion over theimage field, the ratio between Z5 and Z12 being −3, for example, for thefirst lens, and +2, for example, for the second lens, it is thenpossible to set a desired ratio x between Z5 and Z12 by selecting thedeformation of the second lens in the ratio of 3+x/2−x by comparisonwith that of the first lens. The absolute magnitude is governed by thedesired amplitude of Z5 in the wavefront.

Expressed in general terms, the abovenamed ratio is preferably selectedas a function of the influence of the first lens and of the at leastsecond lens on wavefront aberrations of a radially primary Zernike order(for example Z5) and of a radially higher Zernike order (for exampleZ12) thereof.

It is preferred, furthermore, when the first lens and the at leastsecond lens are selected from the plurality of lenses such that aradially primary Zernike order of the image defect can be correctedsubstantially independently of a radially higher Zernike order thereof,or vice versa.

As already mentioned at the beginning, there are linear dependenciesbetween the primary Zernike orders and the radially higher Zernikeorders of the same image defect, such as between Z5 and Z12, forexample, when only one lens is deformed. By suitably selecting thegeometry and/or position of the first and at least second lens, thislinear dependence can be broken with regard to the wavefront aberrationto be corrected.

This can preferably come about by selecting the first and the at leastsecond lens from the plurality of lenses such that the magnitude of thecorrective influence of the first lens on the ratio of the radiallyprimary Zernike order and the radially higher Zernike order isapproximately equal to the corrective influence of the at least secondlens, but has a different sign.

The linear dependence can be broken by the different sign of thecorrective influence of the first lens with reference to the correctiveinfluence of the at least second lens in conjunction with the identicalmagnitude of the corrective influence.

A corrective influence of the first lens and the at least second lens,respectively of different signs, can preferably be implemented by virtueof the fact that the first lens is a positive lens and the at leastsecond lens is a negative lens. The sequence in the direction of thebeam path in the projection objective is not important here, that is tosay the first lens can be arranged in the beam propagation directionupstream of the at least second lens or downstream thereof.

More specific refinements of the method are described below.

When the image defect comprises a combination of field-dependent andconstant-field components or predominantly field-dependent components,there is selected from the plurality of lenses as the first lens and/oras the at least second lens one in the case of which the magnitude ofthe ratio of the subaperture radii of lens front side and lens rear sideis in the range from approximately 0.8 to approximately 0.9 orapproximately 1.1 to approximately 1.2 in the case of a negative lens,and in the range from approximately 0.9 to approximately 1.1 in the caseof a positive lens.

The subaperture radius of a lens is understood as the radius of thelight cone emanating from a field point on the lens front side or thelens rear side. The subaperture radii of lens front side and lens rearside depend on the position of the lens in the projection objective andon their geometry.

In the previously mentioned case that the image defect to be correctedconsists of a combination of field-dependent and constant-fieldcomponents or predominantly field-dependent components, there isselected from the plurality of lenses as the first lens and/or as the atleast second lens one in the case of which the magnitude of the ratio ofthe subaperture radius of lens front side or lens rear side to themaximum lens height is smaller than approximately 0.7 and greater thanapproximately 0.1.

The ratio of the subaperture radius of the lens front side or lens rearside to the maximum lens height is influenced, in turn, by the positionof the lens in the projection objective. Field-dependent components ofan image defect such as, for example, in Z2 (anamorphism) can becorrected most effectively by those actively deformable lenses that arepositioned closer to a field plane than to a pupil plane, as expressedby the previously mentioned value of approximately 0.7.

In the case when the image defect to be corrected has at leastpredominantly constant-field components, there is selected from theplurality of lenses as the first lens and/or as the at least second lensone in the case of which the magnitude of the ratio of the subapertureradii of lens front side and lens rear side is in the range fromapproximately 0.8 to approximately 0.9 or approximately 1.1 toapproximately 1.2 in the case of a negative lens, and in the range fromapproximately 0.9 to approximately 1.1 in the case of a positive lens.

In the case when a positive lens and a negative lens are selected forcorrecting the image defects, the position of these lenses is preferablyselected such that the magnitudes of the beam angles of the marginalrays directly upstream of the positive lens are smaller than the beamangles of the marginal rays directly upstream of the selected negativelens.

It is likewise preferable in this case to select from the plurality oflenses as the first lens and/or as the at least second lens one in thecase of which the magnitude of the ratio of the subaperture radius oflens front side or lens rear side to the maximum lens height is greaterthan approximately 0.7.

Constant-field components are therefore preferably effected by theactive deformation of a lens that is closer to a pupil plane than to afield plane, since elements near the pupil exhibit a substantiallyconstant-field effect on the wavefront aberration in the image field.

For the suitable selection of the first lens and/or the at least secondlens from the plurality of lenses of the projection objective as anactively deformable element or actively deformable elements, it ispreferred to select one in the case of which the ratio of the lenscenter thickness to maximum lens height is smaller than approximately0.4.

Here, the maximum height of a lens is understood as the maximum rayheight in the lens body. This is generally only slightly smaller thanthe real overall height of the lens.

Thus, preference is given, however, to thin lenses as activelydeformable elements, since in the case of these a deformation can beattained by introducing a relatively weak force, and this advantageouslyreduces the outlay on assembly and the requirements placed on themanipulator.

When, however, a thin lens as previously described is not present at asuitable position or with a suitable geometry and instead of this thereis placed at a position suitable for the selection as activelydeformable lens a thick lens that is too thick per se for a deformation,this lens can be split into at least two individual lenses, and at leastone of the individual lenses can be deformed.

In a further preferred refinement, it is provided that there is selectedfrom the plurality of lenses as the first lens and/or as the second lensone that is passed through more than once by the light during operation.

The advantage here is that the manifold, for example two-fold passage ofthe light through such a lens intensifies the optical effect of adeformation of this lens as correction potential, and already virtuallydoubles it in the case of a two-fold passage. A lens through which lightpasses twice during operation is present, for example, in catadioptricprojection objectives with beam deflection.

In a further preferred refinement, the first lens and the at leastsecond lens is deformed with one-fold, two-fold, three-fold or n-foldsymmetry, where n>3.

It is possible in this way for one-fold, two-fold, three-fold or n-foldwavefront aberrations to be at least partially corrected with the aid ofactively deformable lenses in accordance with the previously describedrefinements.

In accordance with a further aspect of the invention, a method isspecified for selecting at least one lens of a plurality of lenses of amicrolithography projection objective as actively deformable element forat least partially correcting an image defect, wherein the geometryand/or position of the at least one lens in the plurality of lensesare/is used as selection criterion as a function of the image defect tobe corrected.

This method is therefore based on the idea of determining in the case ofan existing optics design, or one to be drafted, of a projectionobjective that has a plurality of lenses, at least one, preferably atleast two lenses as specified above that are best suited as activelydeformable lenses for correcting one or more image defects.

It is preferred to use the property of the at least one lens as positivelens or negative lens as selection criterion for a suitable geometry.

The different effects of a positive lens and a negative lens onwavefront aberrations, in particular on the sign (+/−) of the influenceon specific wavefront aberrations, has already been described above.

Furthermore, it is preferred to use the ratio of the center thickness tothe maximum lens height as selection criterion for a suitable geometry.

“Maximum lens height” is to be understood as the maximum ray height onthe relevant lens. This selection criterion takes account of thesuitability of a specific lens of the projection objective to be capableof being deformed in a suitable way by means of a manipulator withoutthe need to operate by introducing strong forces.

It is preferred in this case to select a lens in the case of which theratio of the center thickness to the maximum lens height is smaller thanapproximately 0.4.

A further preferred selection criterion for a suitable position of theat least one lens is the ratio of subaperture radii of the lens frontside and lens rear side.

If the image defect comprises at least predominantly constant-fieldcomponents, there is selected as the at least one lens one in the caseof which the magnitude of the ratio of subaperture radii of the lensfront side and lens rear side is in the range from approximately 0.8 toapproximately 0.9 or approximately 1.1 to approximately 1.2 in the caseof a negative lens, and in the range from approximately 0.9 toapproximately 1.1 in the case of a positive lens.

If the image defect is a combination of field-dependent andconstant-field or of predominantly field-dependent components, there isselected as the at least one lens one in the case of which the magnitudeof the ratio of the subaperture radii of lens front side and lens rearside is in the range from approximately 0.8 to approximately 0.9 orapproximately 1.1 to approximately 1.2 in the case of a negative lens,and in the range from approximately 0.9 to approximately 1.1 in the caseof a positive lens.

A further preferred selection criterion for a suitable position of theat least one lens is the ratio of the subaperture radius of the lensfront side or lens rear side to the maximum lens height.

If the image defect comprises at least predominantly constant-fieldcomponents, there is selected as the at least one lens one in the caseof which the magnitude of the ratio of the subaperture radius of lensfront side or lens rear side to the maximum lens height is greater thanapproximately 0.7.

If the image defect is a combination of field-dependent andconstant-field or of predominantly field-dependent components, there isselected as the at least one lens one in the case of which the magnitudeof the ratio of the subaperture radius of lens front side or lens rearside to the maximum lens height is smaller than 0.7.

A further selection criterion can be whether the at least one lens ispassed through more than once by the light during operation of theprojection objective.

As already mentioned above, such lenses can preferably be selected asactively deformable lenses that are passed through by the light morethan once, for example twice, as can be provided in the case ofcatadioptric projection objectives.

In accordance with a further aspect of the invention, there is provideda microlithography projection objective, comprising a plurality oflenses that are arranged between an object plane and an image plane ofthe objective, wherein a first manipulator for actively deforming afirst lens is assigned to the first lens from the plurality of lenses,the first lens being deformable for at least partially correcting anaberration, at least one second manipulator further being assigned to atleast one second lens from the plurality of lenses, and the at leastsecond lens being deformable in addition to the first lens.

In the case of the projection objective according to the invention, itis possible in accordance with the preferred refinements of theprojection objective that are specified in the claims to apply thepreviously described method in order to improve the imaging propertiesof the projection objective.

Further advantages and features emerge from the following descriptionand the attached drawing.

It is self-evident that the abovementioned features and those still tobe explained below can be used not only in the respectively specifiedcombination, but also in other combinations or on their own withoutdeparting from the scope of the present invention.

The present invention is explained below in more detail with the aid ofselected exemplary embodiments. In the drawing:

FIG. 1 shows a diagram that illustrates wavefront effects in the case ofa combination of two lenses of different deformation, or at differentpositions in the projection objective;

FIG. 2 shows a negative lens and a positive lens for the purpose ofillustrating the different correction effects or wavefront effects of anegative lens and a positive lens when these are actively deformed;

FIG. 3 shows a diagram that illustrates the relationship between theoptical effect of a deformation as a function of the subapertures ofvarious lenses within a projection objective such as is represented inFIG. 11;

FIGS. 4 a) and 4 b) show an exemplary embodiment of a projectionobjective, there being emphasized in FIG. 4 a) specific lenses of theprojection objective that are suitable as actively deformable lenses forcorrecting an image defect or a number of image defects that has or havefield-dependent components, while there are emphasized in FIG. 4 b)lenses of the projection objective that are suitable as activelydeformable lenses for correcting an image defect or image defects thathas or have the overwhelmingly constant-field components;

FIGS. 5 a) and 5 b) show a further exemplary embodiment of a projectionobjective in an illustration similar to FIGS. 4 a) and 4 b);

FIGS. 6 a) and 6 b) show a further exemplary embodiment of a projectionobjective in an illustration similar to FIGS. 4 a) and 4 b);

FIGS. 7 a) and 7 b) show a further exemplary embodiment of a projectionobjective in an illustration similar to FIGS. 4 a) and 4 b);

FIGS. 8 a) and 8 b) show a further exemplary embodiment of a projectionobjective in an illustration similar to FIGS. 4 a) and 4 b);

FIGS. 9 a) and 9 b) show a further exemplary embodiment of a projectionobjective in an illustration similar to FIGS. 4 a) and 4 b);

FIGS. 10 a) and 10 b) show a further exemplary embodiment of aprojection objective in an illustration similar to FIGS. 4 a) and 4 b);and

FIGS. 11 a) and 11 b) show a further exemplary embodiment of aprojection objective in an illustration similar to FIGS. 4 a) and 4 b).

In order in the case of a microlithography projection objective that isconstructed from a plurality of lenses to correct image defects that canoccur on the basis of heating during operation or ageing of the materialof the optical elements, it is provided in the method according to theinvention to select at least two lenses from the plurality of lenses ofthe projection objective and to deform them actively via manipulators inorder at least partially to correct image defects that occur.

Irrespective of whether at least two lenses are selected as activelydeformable lenses, or whether only one lens is selected as activelydeformable lens, a further aspect of the present invention consists inspecifying suitable criteria for selecting such a lens as activelydeformable lens.

The aspect of the present invention mentioned in the first instance andin accordance with which at least two lenses are selected from theplurality of lenses as actively deformable lenses will firstly beexplained in more detail.

If only one lens is used as actively deformable lens for correcting animage defect, only a relatively simple wavefront influence can beproduced by the deformation of the individual lens onto the wavefront inthe image field. If, by contrast, two or more adjacent lenses or lensesarranged at conjugate positions in the system are combined with oneanother such that these lenses have deformations of different sign, forexample on the basis of a different shape of the lenses and/or adifferent introduction of forces by the manipulator or manipulators and,if appropriate, of different sign, for example because of introducingoppositely directed forces, it is possible for there to arise in thecombination of the different profiles of the deformations of the atleast two lenses a complicated wavefront influence such as cannot beattained with the aid of a single deformable lens.

The same result can be achieved when two or more actively deformablelenses are deformed at different positions in the projection objective,these selected lenses being able to have similar deformations, buthaving different wavefront influences owing to the different positionsin the projection objective.

This is illustrated in FIG. 1 for a simple case. Illustrated in the topcurve A is a wavefront influence, assumed to be quadratic, of a firstlens of the plurality of lenses of a projection objective.

Illustrated in the lower curve B is a wavefront influence of an activelydeformed second lens that shows a dependence of fourth power and,moreover, differs from the wavefront influence in accordance with curveA by the opposite sign.

The curve C illustrated with a broken line now shows the super-position,resulting as a sum, of the wavefront influences in accordance withcurves A and B, which shows a more complicated field profile than theindividual wavefront influences of the first lens and of the secondlens, taken alone. It is possible in this way by combining two or morelenses and by appropriate deformation to produce a complicated fieldprofile of the wavefront in order at least partially to compensatewavefront aberrations in the image field.

It is possible in this case, in particular, to deform the first lens andthe at least second lens at a specific ratio to one another. This ratiois selected as a function of the influence of the first lens and of theat least second lens on wavefront aberrations of a radially primaryZernike order and of a radially higher Zernike order thereof.

This is explained by the example of the primary Zernike order Z5 and ofthe radially next higher Zernike order Z12 thereof.

It may be assumed that the first lens produces a wavefront influence inthe case of which the ratio between Z5 and Z12 is −3. It may further beassumed that this ratio is +2 for the second lens. A desired ratio xbetween Z5 and Z12 can then be set by selecting the deformation of thesecond lens at the ratio of 3+x/2−x by comparison with that of the firstlens. The magnitude is governed by the desired amplitude of Z5 (or Z12)in the wavefront.

When using actively deformable lenses that are deformed in one-fold,two-fold or three-fold or higher-fold fashion with respect to symmetryto correct image defects of a projection objective that can arise, forexample, during operation by heating of the optical elements, it is tobe borne in mind that if only one actively deformable lens is used forcorrection the primary Zernike orders and the associated higher Zernikeorders are linearly dependent on one another.

Thus, for example, the image defects having two-fold symmetry inaccordance with the primary Zernike order Z5, and the radially higherZernike order Z12, are dependent on one another, and likewise thethree-fold primary Zernike order Z11 and the three-fold Zernike orderZ20 are dependent, and the four-fold Zernike order Z17 is dependent onthe radially higher four-fold Zernike order Z28.

It is not possible to correct Z12 independently of Z5 on the basis ofthis linear dependence between first and higher orders, for example inthe case of image defects having two-fold symmetry, with the aid of onlyone actively deformable lens, it being possible as a result for one ofthe two Zernike coefficients to rise considerably after correction. Arational optimization is therefore impossible.

A rational correction of such image defects is possible, however, whenat least two lenses are selected for correction purposes as activelydeformable lenses within the projection objective. Of course, it is alsopossible to select more than two lenses as actively deformable lenses,use being made, in particular, of an even number of such lenses. Byusing at least two lenses as actively deformable lenses, there is apossibility of setting primary and radially higher orders independentlyof one another.

In order to ensure such an independence of the correction from primaryand radially higher orders, the corrective influences of the two lensesmust have different signs, but should be similar in terms of magnitude.

It is correspondingly necessary to place specific demands on thegeometry and position of the selected lenses in the projectionobjective.

This results in various selection criteria for the selection of suitablelenses from the plurality of lenses of a projection objective asactively deformable lenses for correction of image defects.

It is possible to achieve a correction that is independent withreference to a first Zernike order and to a radially higher Zernikeorder relevant thereto by making the first lens a negative lens, forexample, and the second lens a positive lens, for example. This isdescribed in more detail hereinafter with reference to FIG. 2.

FIG. 2 illustrates a negative lens 10 and a positive lens 12.

A light ray 14 impinges on a lens front side 16 of the negative lens 10.

Continuous lines illustrate the negative lens 10 in the nondeformedstate, and broken lines illustrate it in the deformed state, which hasbeen brought about by means of a manipulator 21.

The propagation of the impinging light ray 14 inside the negative lens10 takes place in the nondeformed state in accordance with line 18, inthe deformed state in accordance with line 20. After exiting from therear side 22 of the negative lens 10, the light ray propagates furtherin accordance with a line 24 in the nondeformed state of the negativelens 10, and in accordance with a line 26 in the deformed state. 28denotes the front side of the positive lens 12, and 30 the rear side.Continuous lines illustrate the positive lens 12 in the nondeformedstate, and broken lines illustrate it in the deformed state. Deformationof lens 12 is accomplished by manipulator 31. An impinging light ray 32propagates in accordance with lines 34 and 36 in the nondeformed stateof the positive lens 12, and in accordance with lines 38 and 40 in thedeformed state.

It is assumed below that the deformation of the negative lens 10, asalso the deformation of the positive lens 12, is a quadratic function ofthe distance r perpendicular to the optical axis z.

It then holds approximately for the lens thickness d(r) arising afterdeformation that

${{{d(r)} \approx {{d_{0}\left( {1 + {ar}^{2}} \right)}\mspace{14mu}{where}\mspace{14mu} a}} = {\frac{1}{2\; d_{0}}\left( {\frac{1}{R_{H}} - \frac{1}{R_{V}}} \right)}},$

d₀ being the center thickness of the negative lens 10 or the positivelens 12, and R_(V) being the radius of curvature of the lens front side16 or 28, and R_(H) being the radius of curvature of the lens rear side22 or 30.

An n-fold wavefront deformation WFD can be described approximately viathe subaperture radius R_(S) at the respective lens front side 16 or 28,and the respective lens rear side 22 or 30, asWFD≈(1+aR_(S) ²)^(n).

Since the radii of curvature R_(H) and R_(V) are affected by sign, andsince these signs differ from one another correspondingly between thenegative lens 10 and the positive lens 12, is a>0 for the negative lens10 and a<0 for the positive lens 12.

Assuming that the deformation with n-fold symmetry that is imposed onthe negative lens 10 and the positive lens 12 is two-fold (n=2), itfollows approximately for the wavefront deformation WFD that:WFD≈1+2aR_(S) ²+a²R_(S) ⁴.

Turning now to the contributions of the wavefront deformation WFD to theprimary Zernike order Z5 and to the radially higher Zernike order Z12,it holds for the negative lens 10 that:WFD≈a₅Z5+a₁₂Z12, and

for the positive lens 12 that:WFD≈−a₅Z5+a₁₂Z12.

If the deformations are selected such that the contributions a₅ to theZernike coefficient Z5 in the wavefront deformation are equal, thecontributions of the wavefront deformation to Z5 eliminate one another,and the wavefront deformation in the order Z12 can be attackedindependently of the first Zernike order Z5.

Conversely, it is also possible, of course, to proceed in such a waythat the contributions of the wavefront deformation in Z12 eliminate oneanother, and the contributions in Z5 relating to the correction of Z5add up to a finite value such that Z5 can be corrected independently ofZ12 in this case.

In addition to the geometry of the lenses to be selected as activelydeformable lenses inside a projection objective, importance attaches tofurther selection criteria for selecting suitable lenses as activelydeformable lenses that are described hereinafter with reference to FIG.3.

The optical effect of a lens or of a deformation of the latter onwavefront deformations in the image field also depends on the positionof the lens inside the projection objective.

The optical effect of a deformation of a lens, which depends on theposition of the lens inside the projection objective, is influenced atleast inter alia by the ratio of the subapertures at the lens front sideand the lens rear side, as well as by the ratio of the subaperture atthe front or rear side (or the larger of these two) to the maximumheight of the lens.

FIG. 3 shows a diagram that illustrates the relationship between adeformation and the optical effect as a function of the subapertures ofvarious lenses.

The abscissa of the diagram shows the ratio of subaperture (at the lensfront side or lens rear side, or the larger of these two) to the maximumlens height, this ratio naturally being incapable of exceeding 1.

The ordinate of the diagram shows the ratio of the subaperture at thelens front side to the subaperture at the lens rear side.

23 lenses LE1 to LE23 are assigned to the value pairs (R_(SV)/R_(SH);R_(SH)/H_(max)) in the diagram. The lenses LE1 to LE23 belong to theprojection objective, as is illustrated in FIGS. 12 a) and 12 b),respectively. The enumeration of the lenses LE1 to LE23 corresponds tothe sequence of the lenses in the illustration of FIGS. 12 a) and 12 b),from left to right, that is to say from the object plane to the imageplane.

Also plotted in the diagram are lines, each line illustrating anidentical optical effect of a deformation for the value pairs(R_(SV)/R_(SH); R_(SH)/H_(max)).

The direction of the increase in the optical effect is illustrated byarrows A and B.

The optical effect is a measure of the resulting wavefront deformationthat results in the case of a deformation assumed to be identical forall lenses. An increasing resulting wavefront deformation signifies anincreasing optical effect.

Also plotted in FIG. 3 is a frame that illustrates the range of thevalue pairs (R_(SV)/R_(SH); R_(SH)/H_(max)) in which deformations in thelenses present there have the largest optical effect on constant-fieldand field-dependent wavefront deformations. These are the lenses LE11 toLE23 with reference to constant-field wavefront deformations. This rangeincludes lenses for which the ratio of subaperture at the lens frontside or lens rear side to the maximum lens height is larger thanapproximately 0.7, and in the case of which the ratio of the subapertureat the lens front side and the subaperture at the lens rear side is inthe range from approximately 0.9 to approximately 1.1 for positivelenses, and in the range from approximately 0.8 to approximately 0.9 andfrom approximately 1.1 to approximately 1.2 in the case of negativelenses.

It may be inferred correspondingly that in order to correct imagedefects that have overwhelmingly field-dependent components or consistof a combination of field-dependent and constant-field components, thereare selected as actively deformable lenses ones for which the ratio ofthe subaperture to maximum lens height is smaller than approximately0.7, and in the case of which the ratio of subaperture at the lens frontside to subaperture at the lens rear side is in the range fromapproximately 0.9 to approximately 1.1 for positive lenses, and in therange from approximately 0.8 to approximately 0.9 or from approximately1.1 to approximately 1.2 for negative lenses. This is shown in FIG. 3for the lenses LE1 to LE10.

Further selection criteria for lenses to be selected as activelydeformable lenses are the lens thickness, in which case the ratio of thecenter thickness of the lens to lens height should be smaller thanapproximately 0.4, and whether, during operation of the projectionobjective, the light passes through the corresponding lens not onlyonce, but twice or several times, since in the latter case the opticaleffect is virtually doubled or multiplied with each passage.

FIGS. 4 a) to 11 b) illustrate exemplary embodiments of projectionobjectives in the case of which the previously described concepts of thecorrection of aberrations by means of actively deformable lenses areillustrated.

FIGS. 4 a) and 4 b) show a projection objective 50 such as is describedin document DE 102 10 899. The projection objective 50 has a numericalaperture NA of 1.1.

In the following sequence of optically effective modules, the projectionobjective 50 includes in the sense of the passage of light, a first,purely dioptric part of positive refractive power, a biconcave lens thatis arranged in the middle region of the projection objective 50, and athird, purely dioptric part of positive refractive power.

The maximum radius Y′ that an image point can have in the case of thisprojection objective 50 is 11.0 mm.

FIG. 4 a) emphasizes the region of the projection objective 50 in whichfor the lenses present there the ratio of the subaperture at therespective lens to the maximum lens height is smaller than approximately0.7.

In this region, the lenses present there can be selected as activelydeformable lenses for correcting aberrations which are then to beassigned appropriate manipulators M₁, . . . , M_(n) (n≧1), in order atleast partially to correct overwhelmingly field-dependent image defectsor combinations of constant-field and field-dependent image defects (forexample astigmatism on the axis and anamorphism) that can be produced bylens heating or lens ageing.

Illustrated with a dark background in FIG. 4 a) are those lenses thatare suitable as actively deformable lenses for correcting image defects.Those that have no dark background, and thus, to be sure, meet theselection criterion that the ratio of the subaperture to maximum lensheight is smaller than approximately 0.7, are less suited as activelydeformable lenses, for other reasons. For example, the lens L4 is toothick as an actively deformable lens, but in this case the lens L4 couldbe split into two individual lenses, and one of these two individuallenses or both can then be selected as actively deformable lenses.

It is also to be seen from FIG. 4 a) that negative lenses such as, forexample, the lenses L1, L2 and L3 can be selected as actively deformablelenses, and the lenses L7 and L8 can be selected as positive lenses.

The lenses with a dark background in FIG. 4 a) can be used as activelydeformable lenses for correcting those image defects that, as previouslymentioned, have overwhelmingly field-dependent components.

The same projection objective 50 is illustrated in FIG. 4 b), lenses nowbeing illustrated by hatching in the case of which the ratio ofsubaperture at the respective lens to maximum lens height is larger thanapproximately 0.7. These lenses illustrated by hatching, which include,in turn, negative lenses and positive lenses, can be selected asactively deformable lenses for correcting those image defects thatoverwhelmingly have constant-field components, for example Z5, Z11, Z17etc. Appropriate manipulaters K_(n), . . . , K_(m) (m≧1) are providedfor deforming one ore more of the lenses selected for deforming.

FIGS. 5 to 11 feature further exemplary embodiments of projectionobjectives that are to be interpreted in a fashion similar to FIGS. 4 a)and 4 b). The manipulators for deforming the lenses of the objectiveswhich are depicted in FIGS. 5 to 11 with dark background or by hatching,are not shown in FIGS. 5 to 11, but are provided as illustrated in FIGS.4 a) and 4 b).

The projection objective illustrated in FIGS. 5 a) and 5 b) is describedin document WO 2004/019128 A, just like the projection objectiveillustrated in FIGS. 8 a) and 8 b).

In the sense of the passage of light, the projection objectiveillustrated in FIGS. 6 a and 6 b) has in the following sequence ofoptically effective modules a first, purely dioptric part that imagesthe object plane onto a first intermediate image via a first pupilplane, a second, purely catoptric part that images the firstintermediate image onto a second intermediate image via a second pupilplane and that has two concave mirrors upstream and downstream of thesecond pupil plane, respectively, and a third, dioptric part that imagesthe second intermediate image onto the image plane via a third pupilplane. The projection objective in accordance with FIGS. 6 a) and 6 b)has a numerical aperture NA=1.35, the maximum radius Y′ that an imagepoint can have for this projection objective being 16.25 mm.

In the sense of the passage of light, the projection objectiveillustrated in FIGS. 7 a) and 7 b) has in the following sequence ofoptically effective modules a first, catadioptric part that images theobject plane onto a first intermediate image via a first pupil plane, asecond, catadioptric part that images the first intermediate image ontoa second intermediate image via a second pupil plane, and a thirdcatadioptric part that images the second intermediate image onto theimage via a third pupil plane. The numerical aperture of this projectionobjective is NA=1.20, a maximum radius that an image point can have forthis projection objective being 14.4 mm.

The projection objective illustrated in FIGS. 8 a) and 8 b) correspondsto the design principle previously described with reference to FIGS. 7a) and 7 b), this projection objective having a numerical aperture ofNA=1.25, a maximum radius Y′ that an image point can have for thisprojection objective being 15.0 mm.

In the sense of the passage of light, the projection objectiveillustrated in FIGS. 9 a) and 9 b) has in the following sequence ofoptically effective modules a first, dioptric part that images theobject plane onto a first intermediate image via a first pupil plane, asecond, catadioptric part that images the first intermediate image ontoa second intermediate image via a second pupil plane, a third,catadioptric part that images the second intermediate image onto a thirdintermediate image via a third pupil plane, and a fourth, catadioptricpart that images the third intermediate image onto the image via afourth pupil plane. The projection objective has a numerical aperture ofNA=1.30, a maximum radius Y′ that an image point can have for thisprojection objective being 15.75 mm.

The projection objective illustrated in FIGS. 10 a) and 10 b)corresponds to the design principle according to the projectionobjective in accordance with FIGS. 9 a) and 9 b), this projectionobjective having a numerical aperture of NA=0.92, and a maximum radiusY′ that an image point can have for this projection objective being 16.1mm.

The projection objective illustrated in FIGS. 10 a) and 10 b) isdescribed in document JP 2004 317534 A.

Finally, the projection objective illustrated in FIGS. 11 a) and 11 b)has a design principle that was described above with reference to theprojection objective in FIGS. 4 a) and 4 b). This projection objectivehas a numerical aperture of NA=0.95, a maximum radius Y′ that an imagepoint can have for this projection objective being 14.0 mm.

The numerical apertures NA and the maximum radii Y′ that an image pointcan have for the respective projection objective are summarized in thefollowing table:

NA 2Y′/mm FIG. 4a) and 4b) 1.10 22.0 FIG. 5a) and 5b) 1.00 36.0 FIG. 6a)and 6b) 1.35 32.5 FIG. 7a) and 7b) 1.20 28.8 FIG. 8a) and 8b) 1.25 30.0FIG. 9a) and 9b) 1.30 31.5 FIG. 10a) and 10b) 0.92 32.2 FIG. 11a) and11b) 0.95 28.0

The invention claimed is:
 1. A method, comprising: deforming a firstlens of a plurality of lenses via a first manipulator; and deforming asecond lens of the plurality of lenses via a second manipulator, whereina microlithography projection objective comprises the plurality oflenses between an object plane of the microlithography projectionobjective and an image plane of the microlithography projectionobjective; and wherein: a) deformation of the first lens and the secondlens corrects a radially primary Zernike order of an image defectsubstantially independently of a radially higher Zernike order of theimage defect; or b) deformation of the first lens and the second lenscorrects the radially higher Zernike order of the image defectsubstantially independently of the radially primary Zernike order of theimage defect.
 2. The method of claim 1, wherein the first lens and thesecond lens are not adjacent or are arranged at not mutually opticallyconjugate sites between the object plane and the image plane, and bothare deformed such that the deformations exhibit a different deformationprofile.
 3. The method of claim 2, wherein the first lens and the secondlens are of different shape.
 4. The method of claim 2, wherein the firstlens and the second lens are deformed by introducing different forces.5. The method of claim 2, wherein the first and the second lens aredeformed by introducing oppositely directed forces.
 6. The method ofclaim 1, wherein the first lens and the second lens are arranged at notmutually optically conjugate sites between the object plane and theimage plane, and are at least substantially identically deformed.
 7. Themethod of claim 1, wherein the first lens and the second lens aredeformed at a specific ratio to one another.
 8. The method of claim 7,wherein the ratio is selected as a function of the influence of thefirst lens and of the second lens on wavefront aberrations of a radiallyprimary Zernike order and of a radially higher Zernike order thereof. 9.The method of claim 1, wherein the first lens and the second lens areselected from the plurality of lenses such that the magnitude of thecorrective influence of the first lens on the ratio of the radiallyprimary Zernike order and the radially higher Zernike order isapproximately equal to the corrective influence of the second lens, buthas a different sign.
 10. The method of claim 9, wherein the first lensis a positive lens and the second lens is a negative lens.
 11. Themethod of claim 10, wherein the magnitudes of the beam angles ofmarginal rays directly upstream of the positive lens are smaller thanthe beam angles of marginal rays directly upstream of the negative lens.12. The method claim 1, wherein, during operation of themicrolithography projection objective, the light passes through at leastone lens more than once, and wherein the at least one lens comprises amember selected from the group consisting of the first lens and thesecond lens.
 13. The method of claim 1, wherein the first lens and thesecond lens is deformed with onefold, two-fold, three-fold or n-foldsymmetry where n>3.
 14. The method of claim 1, wherein deforming thefirst and second lenses corrects the radially primary Zernike order ofthe image defect substantially independently of the radially higherZernike order of the image defect.
 15. The method of claim 1, whereindeforming the first and second lenses corrects the radially higherZernike order of the image defect substantially independently of theradially primary Zernike order of the image defect.
 16. A system,comprising: a microlithography projection objective, comprising: aplurality of lenses that are arranged between an object plane of themicrolithography projection objective and an image plane of themicrolithography projection objective, the plurality of lenses includinga first lens and a second lens; a first manipulator configured toactively deform the first lens; and a second manipulator configured toactively deform the second, wherein when the first lens and the secondlens are deformed during use of the microlithography projectionobjective: a) a radially primary Zernike order of an image defect iscorrected substantially independently of a radially higher Zernike orderof the image defect; or b) the radially higher Zernike order of theimage defect is corrected substantially independently of the radiallyprimary Zernike order of the image defect.
 17. The system of claim 16,wherein the first lens and the second lens are not adjacent or arearranged between the object plane and the image plane at not mutuallyoptically conjugate sites and are both deformable such that thedeformations exhibit a different deformation profile.
 18. The system ofclaim 17, wherein the first lens and the second lens are of differentshape.
 19. The system of claim 17, wherein the first lens and the secondlens are deformable by introducing different forces.
 20. The system ofclaim 17, wherein the first and the second lens are deformable byintroducing oppositely directed forces.
 21. The system of claim 16,wherein the first lens and the second lens are arranged at not mutuallyoptically conjugate sites between the object plane and the image planeand are at least substantially identically deformable.
 22. The system ofclaim 16, wherein the first lens and the second lens are deformable at aspecific ratio relative to one another.
 23. The system of claim 16,wherein the first and the second lens are selected from the plurality oflenses such that the magnitude of the corrective influence of the firstlens on the ratio of the radially primary Zernike order and the radiallyhigher Zernike order is approximately equal to the corrective influenceof the at least second lens, but has a different sign.
 24. The system ofclaim 23, wherein the first lens is a positive lens and the second lensis a negative lens.
 25. The system of claim 24, wherein the magnitudesof the beam angles of the marginal rays directly upstream of thepositive lens are smaller than the beam angles of the marginal raysdirectly upstream of the negative lens.
 26. The system of claim 16,wherein, during operation of the microlithography projection objective,the light passes through at least one lens more than once, and the atleast one lens comprises a member selected from the group consisting ofthe first lens and the second lens.
 27. The system of claim 16, wherein,when the first lens and the second lens are deformed during use of themicrolithography projection objective, the radially primary Zernikeorder of the image defect is corrected substantially independently ofthe radially higher Zernike order of the image defect.
 28. The system ofclaim 16, wherein, when the first lens and the second lens are deformedduring use of the microlithography projection objective, the radiallyhigher Zernike order of the image defect is corrected substantiallyindependently of the radially primary Zernike order of the image defect.